OBJECTIVE: Muscle strength is a critical clinical hallmark in both health and disease. The current study introduces a novel portable device prototype (MyoQuad) for assessing and monitoring maximal voluntary isometric knee extension torque (MVIT).APPROACH: Fifty-six patients with inclusion body myositis were studied. Knee extension weakness is a key feature in this inflammatory muscle disease. Cross-validation with an isokinetic dynamometer (Biodex System 3 Pro) was performed. Between-day reproducibility and ability to monitor changes in muscle strength over time compared to the gold standard method as a reference, were also investigated.MAIN RESULTS: The measurement was feasible even in the weakest patients. Agreement between methods was excellent (standard error of measurement (SEM) was 3.8 Nm and intra-class correlation coefficient (ICC) was 0.973). Least significant difference (LSD) was 4.9 and 5.3 Nm for the MyoQuad and the Biodex, respectively Measurements using the MyoQuad exhibited excellent between-day reproducibility (SEM was 2.4 Nm and ICC was 0.989 versus 2.6 Nm and 0.988 using the Biodex). Changes in MVIT at 6 and 12 months were similar between methods (timepoint  ×  method interaction was not significant; all p   >  0.19); strength changes classified according to LSD at 6 and 12 months were consistent between methods (>70% consistent classification)).SIGNIFICANCE: The measurement of MVIT using the MyoQuad offers a cost-effective, portable and immediate alternative for the routine measurement of maximal voluntary isometric strength of the quadriceps. The MyoQuad offers a comfort and stability that cannot be provided by standard hand-held dynamometers. These results support quantitative muscle strength assessment using fixed yet flexible dynamometry within clinical routine and multicenter trials.